This invention relates to the structure and fabrication of small tanks or vessels to be used for the transport, storage or utilization of pressurized fluids. More specifically, it is the purpose of this invention to provide a small tank formed of multiple side by side basically cylindrical lobes, which are joined in a nearly rectangular overall configuration to provide a tank having the volumetric capacity of present gasoline or diesel automotive or motor vehicle fuel tanks, together with the strength characteristics of cylindrical tanks to accommodate pressurized fluids. Relatively large multi-lobed tanks for the transport of pressurized fluids in marine vessels, or for the land storage of such pressurized fluids are shown and described in Secord U.S. Pat. No. 4,182,254 for "Tanks for the Storage and Transport of Fluid Media Under Pressure." It would be desirable to extend this basic technology to enable relatively inexpensive and commercially feasible fabrication of small tanks, such as automotive fuel tanks, which are capable of handling fluids under pressure. The pressure containing capability of such a multi-lobed tank would permit its use for compressed gases, such as natural gas or hydrogen, or for liquefied gases, such as propane or butane. In general, pressurized fuel systems eliminate the problems associated with fuel evaporation or the need for vapor recovery during filling, both of which are recognized concerns of environmental protection. Moreover, the multi-lobed configuration would enable the fabrication of a near-rectangular vessel that could be accommodated in the space presently required for motor vehicle gasoline or diesel fuel tanks.
It is generally accepted knowledge that the hoop forces generated in a thin walled cylindrical vessel, as a result of internal pressure, are always tensile and tangential, and that the hoop stress in the wall of a cylindrical tank under pressure is proportional to the internal pressure, as well as to the radius of curvature of the cylindrical vessel.
Of course, in the case of a rectangular vessel with planar surfaces, the radius of curvature is infinite and, therefore, the stress is also infinite, even at a very low internal pressure. In actual practice, there is some deflection of the flat surface and the stress is actually finite. However, it is of considerable magnitude, and the ability of a rectangular vessel, or any vessel with flat walls, to withstand internal pressure is minimal. Therefore, tanks for the storage and/or transportation of pressurized fluids are generally made cylindrical or spherical. The multi-lobed vessel illustrated and described in the aforesaid U.S. Pat. No. 4,182,254 provides a means of approaching the volumetric economy of a rectangular tank, while limiting the working stresses of the vessel to that of a cylindrical tank with a modest radius of curvature.